US and Russia at odds over UN Security Council vote on renewing an inquiry into chemical weapons use in Syria.

The future of a UN inquiry into chemical-weapons use in the war in Syria remains uncertain, with opposing Security Council resolutions backed by the US and Russia being proposed.

The mandate of the Joint Investigative Mechanism (JIM), an inquiry launched jointly by the UN and the Organization for the Prohibition of Chemical Weapons in 2015, was set to expire at midnight New York time on Thursday.

A vote on a US-sponsored Security Council resolution to continue the inquiry's work was scheduled for Thursday afternoon.

US President Donald Trump said on Twitter that the Security Council must extend the JIM's mandate to ensure President Bashar al-Assad's government in Syria "does not commit mass murder with chemical weapons ever again".

Need all on the UN Security Council to vote to renew the Joint Investigative Mechanism for Syria to ensure that Assad Regime does not commit mass murder with chemical weapons ever again.

It also said the Islamic State of Iraq and the Levant (ISIL) group was responsible for releasing sulpur mustard gas in an attack in Umm Hawsh in September last year.

Two women were exposed to mustard gas in that incident, after a mortar shell containing the substance hit a house and another shell was found lodged in the pavement, the UN said.

"There has been sufficient evidence of a credible and reliable nature to make its findings," said Edmond Mulet, who heads the JIM, during a briefing on the report's findings.

Though the UN said it was "too dangerous" to visit Umm Hawsh and Khan Sheikhoun, the committee said it had gathered enough information to make a solid conclusion, according to a UN statement.

Mulet said the JIM conducted its work "in an independent, impartial and professional manner".

Russian veto

Russia vetoed an earlier resolution sponsored by the US to extend the JIM's work for an additional year, saying at the time that it wanted to wait for the release of the JIM's report on chemical weapons use by the Syrian government.

Eleven Security Council member states voted in favour of that resolution in late October, while Russia and Bolivia voted against it and China and Kazakhstan abstained.

Russia has vetoed nine resolutions on Syria since the conflict started in 2011, according to Reuters.

In a joint statement released earlier this month, Rex Tillerson, US secretary of state; Jean-Yves Le Drian, French foreign minister; Boris Johnson, UK foreign secretary; and Sigmar Gabriel, German foreign minister, condemned the use of chemical weapons by both the Syrian government and ISIL, also known as ISIS.

"We agree that it is vital for the international community to continue to investigate cases where chemical weapons have been used in Syria," they said.